The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.
The present invention relates generally to lasers and more specifically to a diode laser-pumped magnetometer. Magnetometers have many applications in a variety of commercial, research, and military areas. The range of scale sizes of magnetic mapping is very large. An example of a small scale application might be for non-destructive evaluation of a weld in a nuclear reactor containment shield. Large scale application would include mapping the earth""s magnetosphere, or surveying for offshore oil reserves. Particularly interesting opportunities for magnetometers include: sensors for medical or biomagnetic brain cortex imaging, sensors for detection of underground weapons production facilities, and sensors for extremely low frequency (ELF) communication receivers. Development of more sensitive, faster sampling, vibration tolerant magnetometers will benefit all application.
The task of producing a diode laser-pumped magnetometer is alleviated, to some extent by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 3,750,008, Jul. 31, 1973, Optical pumping magnetometer, Asano, Hiroshi,
U.S. Pat. No. 3,652,926, Mar. 28, 1972, Optical pumping magnetometers, Brun, Henri,
U.S. Pat. No. 5,602,475, Feb. 11, 1977, Laser pumped magnetometer, Bohler, Christopher,
U.S. Pat. No. 5,493,223, Feb. 20, 1996, Resonance magnetometer with optical pumping using a monolithic laser, Leger, Jean-Michel,
U.S. Pat. No. 4,806,864, Feb. 21, 1989, Laser-pumped helium magnetometer, Schearer.
The primary goal of this invention was to demonstrate the feasibility of using a miniature diode laser as a pump source for optically pumped magnetic resonance alkali vapor magnetometers. As we show later, we were successful in these demonstrations, and our Phase I results indicate that diode laser pumped alkali magnetometers will have significant advantages over conventional magnetometers. First we note that in general, optically pumped magnetometers (OPM) have several advantages over magnetic flux magnetometers. Optically pumped alkali magnetometers are:
Essentially unaffected by the direction of the magnetic field,
Inherently insensitive to vehicle or platform vibration,
Capable of long baseline normalization.
In particular, a fiber-optic coupled diode laser magnetometer offers numerous significant improvements over current instruments relying on discharge resonance lamps.
a) Vapor cells can be made smaller for laser instruments. This translates into better sensitivity because the uniformity of the magnetic field is typically higher over small scale sizes.
b) Smaller vapor cells require smaller rf coils reducing rf field inhomogeneities which may translate into narrower resonance linewidths, better sensitivity, and smaller dead zone angles.
c) The additional light intensity available with diode lasers may generate enhanced polarization in the atomic vapor and this may generate an order of magnitude increase in magnetometer sensitivity.
d) Because fiber optics are non-magnetic, a fiber optic coupled device allows flexible location of the instrument without magnetic interferences. For example the diode laser could be concurrently located in the body of the aircraft and utilize a fiber to deliver the light to several magnetometer sensor heads in a tether unit or at the aircraft wing tip. Fiber optic bifurcation and splitting, even tenfold, is now routine.
e) Resonance lamps are not easily or efficiently coupled to fiber optics due to the optical divergence of the lamp light. The low divergence beam of a diode laser easily couples to fiber optics and could provide sufficient light intensity to operate many magnetometers simultaneously.
f) Fiber optic diode laser technology has the potential to be battery operated at room temperature, miniaturized, and ruggedized for development of a covert magnetometer package. Existing SQUID devices require cryogen to be replaced every few days, and small resonance lamp magnetometers are inefficient power consumers.
The present invention includes a diode laser-pumped magnetometer made up of a diode laser, K-cell, and photo sensor. The diode laser emits a polarized pumping laser beam with resonant optical radiation. This pumping laser beam is sent to a K-cell through which a magnetic field is manifested from an independent source. The K-cell contains atoms with a dipole experiencing a torque due to the magnetic field. These atoms are excited by the resonant optical radiation of the pumping laser beam and periodically emitting a response radiation as they return to ground state such that the response radiation includes photons that indicate one unit of angular momentum indicative of the torque due to the magnetic field; and
Finally a photodiode and scope act as a means for measuring the response radiation of the K-cell to indicate thereby a measure of the magnetic field in the K-cell from the independent magnetic source.